The present disclosure relates to a compression limiter retainer arrangement for a plastic component, and more particularly, to a compression retainer arrangement for a plastic chain guide for an internal combustion (IC) engine.
More stringent fuel economy regulations for the transportation industry have prompted the need for improved vehicle efficiencies. Many focused design efforts that contribute to the achievement of future regulatory requirements are in process. Two such efforts are friction reduction and lightweighting, both of which are being scrutinized for many vehicle components. Regarding the IC engine that powers most of today's on-highway vehicles, friction reduction has made large strides in recent years with improvements to many of the rubbing interfaces, including the piston interface with the piston bore (coatings, piston ring tension) and instances where hydrodynamic bearings are being replaced with rolling element bearings (camshafts, balance shafts). Lightweighting involves the process of optimizing the design of a component through finite element analysis (FEA) and can often employ a change in material. In some instances, lightweighting can also lead to friction reduction as weight reduction can lead to dynamic load reduction of a given component, which can facilitate lower loads between rubbing interfaces.
Chain drive systems are often used on an IC engine to drive the camshafts, balance shafts or oil pump. Camshaft drives transmit the rotary motion of the crankshaft at a 2:1 ratio to one or more camshafts in either an overhead valve or overhead camshaft valve configuration. As the camshafts rotate, intake and exhaust valves are opened and closed by lobes on the camshaft that interface with the valve train of the engine. The timing of the intake and exhaust valve events, relative to the location of the piston within the combustion cycle is essential to the function of the engine as is the ability to maintain this precise timing over the lifetime of the engine. Balance shaft drives transmit the rotary motion of the crankshaft typically at either a 1:2 or 1:1 ratio to one or more balance shafts. A balance shaft consists of an eccentric-weighted shaft that, when rotated, imparts a force on the engine to counteract vibrations due to the inherent imbalance of some engine cylinder arrangements. The rotational position of the eccentric weight of the balance shaft with respect to the piston position within the combustion cycle is essential to facilitate a smoother running engine. Oil pump drives transmit the rotary motion of the crankshaft to an oil pump that generates oil pressure for the lubrication system of the engine. In some engines, the oil pump and balance shafts are integrated into a single component which requires only a single drive input from the crankshaft.
The crankshaft of an IC engine is continuously accelerated and decelerated throughout its rotation due to the four strokes of a combustion cycle (intake, compression, power, and exhaust) combined with the firing order of the engine. Given the fact that a chain drive typically derives its rotary power from the crankshaft and that each driven component (camshafts, balance shafts, oil pump) has its own unique load characteristic, a chain drive system must be capable of controlling unique drive system dynamics. Special chain drive components exist to accomplish this task such as a tensioner, tensioner arm and a fixed chain guide. The tensioner arm and fixed chain guide come into direct contact with the chain and are often made of plastic to provide a low friction and low noise interface. In addition, the use of a plastic component instead of a metal component offers significant weight advantages.
The design of the attachment points of a plastic component to an IC engine should allow for: 1). manufacturing tolerances of the plastic component attachment points, 2). manufacturing tolerances of the engine structure attachment points (for example, threaded holes), and 3). environmental factors such as humidity and temperature that can thermally expand or contract a plastic component. To address these three factors in plastic components that contain two attachment points, one of the attachment points can be a round hole and the other attachment point can be a slotted or oblong hole. Such an attachment strategy is often found in plastic chain guides within a chain drive system.
Attaching a plastic component to an IC engine often requires use of a metal component at the attachment points. Often termed compression limiters, these metal components can eliminate the risk of over-stressing the plastic by a fastener during the installation process or eliminate the creep effects of plastic material at the fastener interface which can reduce the clamp load of the fastener. The metal compression limiter can take many forms including a cylinder or a flanged cylinder, with the flange portion interfacing with the fastener. Many engine manufacturers require that the compression limiter is preassembled with the plastic component by the engine component supplier before delivery to the engine assembly plant. This can be accomplished in multiple ways such as overmolding the metal compression limiter within the plastic component; or, in the case of a slotted or oblong hole, it can be retained by design features within the plastic component that allow movement of the compression limiter in order to become aligned with the fastening point on the IC engine during the assembly process.
Packaging space within today's IC engines is very limited and can be a significant design challenge for an engine component. Tool clearance to fasten the component to the engine can add to this packaging challenge. For a plastic engine component, such as a chain guide that is typically long and slender, design features that retain the compression limiter are often located on the portion of the fastener hole that is furthest away from the chain interface to avoid locating any structural irregularities that can reduce the rigidity and/or increase internal stresses such that the load bearing capacity is reduced. This typically requires extra material around the attachment holes furthest away from the load interface which increases the size of the component. In some engine applications the resultant larger size of the chain guide will interfere with adjacent components. A design solution for a plastic chain guide is needed to incorporate a compression limiter retention feature that minimizes the packaging sizes without exceeding critical stress levels.